Probably send an email or call. It would help them out clarify why you didn't respond. I'm not sure, but if your date passes and school's don't hear from you, they might waste their time trying to contact you to make sure you don't want a seat (eg your letter got lost in the mail, you didn't realize you had a deadline, still coming up with the deposit $, etc etc). If you tell them up front you're going somewhere else, there is no confusion.
It's much more courteous to call/email the school declining the offer. They took the time to accept you, so you should take the time to decline politely.
It's much more courteous to call/email the school declining the offer. They took the time to accept you, so you should take the time to decline politely.